[GHC] #5412: dataTypeConstrs gives unhelpful error message

[GHC]

#5412: dataTypeConstrs gives unhelpful error message
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        Reporter:  NeilMitchell      |            Owner:  klangner
            Type:  bug               |           Status:  patch
        Priority:  normal            |        Milestone:  7.6.2
       Component:  libraries/base    |          Version:  7.1
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by klangner):

 Yes you are right. The problem is that this kind of message can't be used
 in other places. So I was not sure if I should change other functions as
 well.
 On the other hand it would be nice to improve some other error messages
 since probably sooner or later this will return as new ticket.

 How do you think would be better?
 1. Change only function from this ticket description
 2. Change only messages where ADT is expected (4 places) and leave other
 messages intact.
 3. Try to improve other error messages in this module as well.

 Third option will require more work and I'll probably need some help with
 the message texts.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5412#comment:12>
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